Fabrication of structural armor

ABSTRACT

Fabrication techniques for and examples of metallic composite materials with high toughness, high strength, and lightweight for various structural, armor, and structural-armor applications. For example, various advanced materials based on metallic-intermetallic laminate (MIL) composite materials are described, including materials with passive damping features and built-in sensors.

This application is a divisional of U.S. patent application Ser. No.11/629,578, filed on Oct. 30, 2007, which is a national stageapplication of and claims the benefit of PCT Application No.PCT/US2005/021095 filed on Jun. 15, 2005. PCT Application No.PCT/US2005/021095 filed on Jun. 15, 2005 claims the benefit of U.S.Provisional Patent Application No. 60/580,867, entitled “DESIGNS ANDFABRICATION OF STRUCTURAL ARMOR”, and filed on Jun. 17, 2004. Thedisclosure of the prior applications is considered part of (and isincorporated by reference in) the disclosure of the this application.

BACKGROUND

This application relates to structural armor materials, their designs,fabrication, and applications.

Structural armor materials are specially designed to exhibit highmaterial strengths and resistance to ballistic impacts. Such materialsmay be used to protect persons and various objects such as motorvehicles, aircraft, and buildings, from ballistic and other harmfulimpacts. One type of structural armor materials use multi-layercomposite structures of different material layers to form metallicintermetallic laminate (MIL) composites. MIL composites may be designedto be relatively light in comparison to various other armor materials.U.S. Pat. No. 6,357,332, for example, describes examples of MILcomposite armors and associated fabrication processes.

SUMMARY

This application includes, among others, structural designs andfabrication of metallic materials based on metallic-intermetalliclaminate (MIL) composite materials.

In one implementation, a metal box is provided to have an opening and ametal lid plate for closing the opening. A stack of alternating firstmetal and second metal layers is placed inside the metal box. A firstmetal in the first metal layers and a second metal in the second metallayers are operable to react under heat and pressure to form anintermetallic material. The opening is then closed by the metal lidplate to compress the stack inside the box to contact each inner metalwall of the box. Pressure and heat are then applied to the closed metalbox to compress the stack and to cause reaction between two adjacentlayers in the stack and reaction between the stack and each inner metalwall of the box to form metallurgical bonding between adjacent layers inthe stack and between the stack and the metal box.

In another implementation, a substrate made of a first metal is providedto include a surface. A metal sheet made of a second metal is thenplaced on the substrate in contact with at least a portion of thesurface. Pressure and heat are applied to the substrate and the metalsheet to compress the metal sheet against the surface to cause reactionbetween the metal sheet and the surface and to form an intermetalliccompound.

An article of manufacture is also disclosed as an example. This articleincludes a stack of alternating metal and intermetallic layersmetallurgically bonded to one another, and cavities in at least oneintermetallic layer and filled with a filling material. Each metal layerincludes a first metal and each intermetallic layer includes an alloy ofthe first metal and a second metal.

In another example, an article of manufacture is described to include ametal substrate and a stack of alternating metal and intermetalliclayers metallurgically bonded to one another and to a surface of themetal substrate. Each metal layer includes a first metal and eachintermetallic layer includes a compound of the first metal and a secondmetal. The thickness values of the layers in the stack are spatiallygraded.

In yet another example, an article of manufacture may include asubstrate including a first metal and a stack of alternating metal andintermetallic layers metallurgically bonded to one another and to asurface of the substrate. Each metal layer includes the first metal andeach intermetallic layer includes a compound of the first metal and asecond metal. The article further includes metal wires penetratingthrough the stack and each having a portion embedded in the substrate.Each metal wire is metallurgically bonded to the stack and substrate.

In yet another example, this application describes a structure thatincludes a stack of alternating metal and intermetallic layers and atleast one sensor embedded in the stack. The layers are metallurgicallybonded to one another. Each metal layer includes the first metal andeach intermetallic layer includes a compound of the first metal and asecond metal. The sensor is operable to measure a parameter indicativeof a condition of the stack.

Furthermore, this application describes an article of manufacture thatincludes a stack of alternating metal and intermetallic layersmetallurgically bonded to one another, and a closed metal box enclosingthe stack. Each metal layer comprises a first metal and eachintermetallic layer comprises an alloy of the first metal and a secondmetal. Each inner wall of the closed metal box is metallurgically bondedto the stack.

These and other implementations are described in greater detail in theattached drawings, the detailed description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C illustrate examples of metallic-intermetalliclaminate (MIL) composites.

FIG. 2 shows an exemplary system for manufacturing a MIL compositematerial.

FIG. 3 is a chart that compares material properties of differentmaterials including MIL composites materials exhibiting large specificcompressive strengths.

FIGS. 4A and 4B illustrate an example of a MIL composite material havingcorrugated layers.

FIGS. 5A and 5B show an example of confined MIL composite material.

FIGS. 6, 7, 8A, 8B, 8C, 9A, 9B, and 9C show examples of substratescoated with MIL composite layers.

FIGS. 10A, 10B, and 10C show an example of a MIL composite materialhaving passive vibration-damping cavities.

FIGS. 11A and 11B show examples of MIL composite materials havingembedded sensors.

FIGS. 12A and 12B illustrate one implementation of an embedded vibrationsensor in a MIL composite material.

DETAILED DESCRIPTION

Various metallic-intermetallic laminate (MIL) composites are known fortheir material strengths, especially at high temperatures. Typically, aMIL composite may be fabricated from pressing a stack of interleavingmetal layers made of a first metal with a high toughness and beingductile, and a second metal suitable for reacting with the first metalto form an intermetallic compound at a high temperature. The resultantMIL layer exhibits a ceramic-like material properties such as a highmaterial strength and a high material stiffness. The final laminatedcomposite material essentially retains the ceramic-like strength andstiffness of the MIL layers and the toughness and some of the ductilityof the first metal. The processing under the high temperature may becontrolled to cause the second metal to completely react with the firstmetal so that the final structure is essentially a fused or laminatedstack of MIL layers interleaved with remaining first metal layers. SuchMIL composite materials may be used as armor layers to resist ballisticimpacts and to protect persons, animals, and various objects. Someexamples of such MIL composite materials are described in U.S. Pat. No.6,357,332, the entire disclosure of which is incorporated by referenceas part of the specification of this application.

For example, MIL materials may be made from titanium and aluminum toform a Al₃Ti/Ti intermetallic compound with excellent material strength,stiffness, and toughness. Notably, this material can have a highcorrosion resistance and are light in weight. The titanium metal with ahigh toughness may be replaced by other metal materials with a hightoughness. Examples include a titanium alloy, nickel, a nickel alloy,vanadium, a vanadium alloy, iron, an iron alloy, tantalum, a tantalumalloy, and any combination of two or more materials selected fromtitanium, nickel, vanadium, iron, tantalum and their alloys, or anyother metal that forms aluminides. The aluminum may be replaced by analloy of aluminum, an aluminum metal-matrix composite, analuminum-infiltrated ceramic composite, or all together replaced bymagnesium, magnesium alloys, a magnesium metal-matrix composite, or amagnesium-infiltrated ceramic composite. In the situation of the secondmetal being magnesium-based, the first metals can now includealuminum-based metals and composites. When the second metal ismagnesium-based, the intermetallic phase will be a compound of magnesiumand the first metal. The example of the MIL composites being based ontitanium and aluminum will be used throughout this patent, but it isunderstood that these metals can be replaced by any of the first andsecond metals described above.

FIG. 1 illustrates an exemplary initial stack of alternating metallayers 110 and 120 for a MIL composite material prior to the pressingfabrication under a high temperature. The metal layers 110 are made of afirst metal of a high toughness such as titanium. The metal layers 120are made of a second metal such as Al or its alloy. After the properpressing, the first and second metals react with each other andsacrifice the part of each first metal layer 110 and the substantiallythe entire second metal layer 120 to produce a composite layer 130sandwiched between two remainder layers 110A of the first metal layers110. FIG. 1B illustrates the structure of the MIL composite materialafter the fabrication process.

FIG. 1C further shows an example of a Al₃Ti/Ti MIL composite materialfabricated by hot pressing through a load-temperature cycle in the air,without utilizing any kind of inert or protective atmosphere. Theresultant intermetallic, Al₃Ti, possesses a high strength, a highYoung's modulus (e.g., 216 GPa), a low density (e.g., 3300 Kg/m3) and alow ductility. The thickness values of the initial titanium and aluminumfoils are controlled such that the final composite consists of Al₃Ti aswell as un-reacted titanium. The Ti/Al₃Ti MIL composites show excellentspecific mechanical properties (such as fracture toughness) that canrival those of conventional metals and ceramics. The high toughness ofthe MIL composites is primarily attributed to the alternate layering ofbrittle and ductile layers. The ductile phase reinforcement of brittlematerials utilizes crack-laminate interactions to generate a zone ofbridging ligaments that restrict crack opening and growth by generatingclosure tractions in the crack wake and utilize the work of plasticdeformation in the ductile metal phase to increase fracture resistanceof the composite. Thus, a crack propagating in the brittle Al₃Ti layersis effectively stopped every time it hits the ductile titanium layer.

FIG. 2 illustrates one exemplary hot pressing system for making MILcomposite materials in the air without a gas-buffered environment. Thissystem allows for fabrication of MIL composites in open air has a numberof advantages. For example, vacuum or inert atmospheres generallyrequire greater apparatus cost and processing time, and may also limitthe overall size of the samples that can be produced. The open airfabrication removes these and other limitations.

In one implementation of the fabrication process by reacting under heatand pressure, the interleaved first and second metal layers are pressedunder pressure. The operating temperature is then raised to atemperature less than a melting point of the one or more second metalsand metal alloys but sufficiently high so that, at pressure, the solidstate diffusion occurs between the interleaved layers, physicallyengaging and locking the layers in place. The temperature of thepressured, diffused, locked interleaved layers is raised until all theone or more second metals are reacted with the one or more first metalsto form an intermetallic compound. In this process, the temperature israised at a sufficiently slow pace and under sufficient continuingpressure so that, despite the fact that the reacting proceeds withincreasing difficulty and an ultimate high temperature reached isgreater than a melting point of the one or more second metals, the oneor more second metals remain initially locked in place and ultimatelybecome reacted without squirting in liquid state from between the firstmetal foils. Next, the material is cooled to the room temperature toform the final structure which includes layers of one or more firstmetals and metal alloys that are interspersed with regions of a hardintermetallic compound. Notably, each step transpires in the open airwith the presence of atmospheric gases. The second metal layers becomesubstantially or completely reacted with the first metal layersnonetheless that the temperature of liquefaction of the at least onesecond metals and metal alloys from which the second metal layers aremade is exceeded during the process.

In the example of the Al₃Ti/Ti composite materials, the diffusion andreaction between titanium and aluminum to form the intermetallic phaseAl₃Ti exhibit different behaviors at temperatures significantly belowand above the melting point of aluminum (660° C.). At temperatures belowthe melting point of aluminum, e.g., from 400° C. to 642° C., Al is themajor diffusing species. The diffusion of Al is not affectedsignificantly by an interfacial oxide layer, but an interfacial oxidelayer reduces the nucleation rate of Al₃Ti. Growth of the Al₃Tiintermetallic tends to occur exclusively on the Ti-rich side with asmall fraction of Al inclusions, and linear kinetics are observed untilthe breakdown of the oxide layer, after which parabolic intermetallicgrowth rates are observed. When oxide films are present between metals,linear kinetics are seen in the early stages of diffusion and laterbecome parabolic. The reaction layer formed is composed of Al₃Tiparticles in an aluminum matrix, and solid solutions are generallyabsent.

As an example, foils of commercial purity 1100 aluminum and Ti-3Al-2.5Vfoils may be stacked in alternating layers and are processed in thecomposite synthesis apparatus in FIG. 2. The foil dimensions may beselected to completely consume the aluminum in forming the intermetallicwith alternating layers of partially un-reacted Ti metal. Foils may becleaned in a bath of 2 pct HF in water, rinsed in water, and then rinsedin methanol and rapidly dried in order to remove oxide layers andsurface contaminants before processing.

Next, the cleaned foil stack is placed between two cartridge-heatednickel alloy platens and placed on the crosshead of a screw-driven loadframe. The synthesis apparatus may be surrounded by ceramic fiberblanket material to reduce heat loss. After foil loading, the pressureis increased to about 3.8 MPa by load control at room temperature toensure good contact between foils. The temperature is initially raisedto 600-650° C. for 2 to 3 hours, while maintaining the pressure, toallow diffusion bonding of the layers and, thus, minimize internaloxidation between the layers. The temperature is then slowly raisedthrough the melting temperature of the second metal and the pressure isreduced to about 2.3 MPa (to eliminate expulsion of liquid phases). Thetemperature is further raised above the melting point of the secondmetal, where the pressure drops to about 1.5 MPa as a result of thereaction as liquid phases form over a 2 to 3 hour period; the pressurewill then increase as a result of intermetallic solidification to about3.5 MPa when the bulk of the reaction is complete. The temperature isthen increased slowly to 670° C. or above to ensure the reaction hasreached the corners of the sample. The sample is then air-cooled whilemaintaining the pressure at about 3.8 MPa.

The Ti—Al₃Ti MIL composites may have the specific stiffness(modulus/density) nearly twice that of steel, the specific toughness andspecific strength similar or better than many metallic alloys, andspecific hardness close to many ceramic materials. FIG. 3 shows acomparison of different armor materials. The x-axis is the specificmodulus of a material on a log scale and the y-axis is the specificcompressive strength on a log scale. Good armor materials should belocated in the upper right-hand corner of the plot. The location of theMIL composites (red ellipse) is shown to the right (higher specificmodulus) of the typical structural metals such as steels, Ti alloys,Ni-superalloys, Al-alloys, and Ti-based intermetallics. The onlymetallic materials of higher specific stiffness shown are the berylliumalloys. Several ceramic materials are also shown to have higher specificstiffness than the MIL composites, including SiC, B₄C, Al₂O₃, anddiamonds. Clearly, the MIL composites possess has superior materialproperties for structural applications such as applications demandinghigh specific stiffness combined with high fracture resistance.

The above examples of MIL composite materials use planar layers.Alternatively, the composite layers may be nonplanar and include certaincontours or geometries to enhance the structural performance. Forexample, the metal layers used in a MIL composite material may havecorrugations or corrugated features. Such metal layers are stacked andthen subjected to heat and pressure to form the MIL structure. Thecorrugations in different metal layers may be identical to one anotherand may be different. The corrugations in different metal layers may bespatially shifted relative to one another. Prior to the processing underheat and pressure, the stacked layers may have air pockets or voidsbetween different layers due to the presence of the corrugations. Uponprocessing, all layers are fused together into a solid compositematerial.

FIG. 4A illustrates fabrication of identically corrugated plates 411 and412 using a pair of corrugated pressing plates 410 and 420. In thisexample, planar plates of two different metal materials are interleavedto form a stack. This stack of planar plates is then pressed between thepressing plates 410 and 420 to become corrugated plates with identicalcorrugated patterns under heat and pressure to form a corrugated panelwith a MIL structure. Multiple corrugated panels may be used to form afinal structure.

FIG. 4B shows one exemplary structure formed from four identicalcorrugated panels that are parallel in their corrugated directions andare spatially shifted to misalign the corrugated patterns. The space orvolume 430 between two adjacent panels may be filled with a suitablemetallic material, such as a metal foam, air, fire retardant,penetration resistant fibers, or any number of materials. Alternatively,successive corrugated panels of the composite laminate material may alsobe aligned with their corrugations in an orthogonal orientation.

A corrugated panel is load-bearing in one of its two planar axis, andthat several spaced-parallel corrugated panels may suitably bear highloads within, as well as transversely to, their substantial planes.Accordingly, arrayed composite laminate panels are suitable for goodconstruction materials, such as for the sides of armored fightingvehicles, aerospace structures and for buildings.

The above and other corrugated composite laminate materials benefit allmechanical and strength advantages associated with corrugation. Forexample, a corrugated panel may be capable of better supporting a loadaligned with axis of corrugations in the plane of the material withoutbuckling or bending. To this extent the utility of the material forconstruction, including for load-bearing walls and the sides of armoredvehicles, is enhanced. As another example, the corrugations help to turnthe path of an impacting projectile. To account for the statisticallysmall probability that the projectile should hit centrally in the troughof a corrugation, it is possible to back one panel of corrugated armorwith another, offset, panel. If structural strength is desired in twoperpendicular directions in the plane of a composite laminate materialdescribed, then corrugated panels of the material having theircorrugations running in one direction may be alternated with otherpanels of the material having their corrugations running at a 90-degreeangle.

The above planar and corrugated MIL composite materials may be used toconstruct various advanced materials for structural, armor, andstructural-armor applications. For example, a MIL composite material maybe metallurgically bonded to and confined by a metallic box to form aconfined MIL composite structure. As another example, the MIL compositelayer structure and the associated processing may be used to fabricate asurface layer over a metallic substrate or plate provide a hardprotection layer that is resistant to wear and corrosion. In addition,the MIL composite materials may be designed to further harden it byembedding localized materials that are spatially distributed within aMIL composite material. A MIL composite material may also be designed toinclude, at selected locations, built-in cavities with loose powder andother suitable materials to create an internal vibration-dampingmechanism. Furthermore, a MIL composite material may also be designed asan “intelligent” material to include sensors at selected locations tomeasure and monitor a parameter of the material at these selectedlocations, such as the magnitude of the impact to the material, thetemperature, and other measurable parameters. Examples andimplementations of these and other MIL-based materials are now describedin the following sections.

FIGS. 5A and 5B illustrate an exemplary confined MIL material in which aMIL composite material 501 is bonded to and confined within the metallicwalls of a metallic box. Notably, the bonding between the MIL compositematerial 501 and the metallic walls of the box or container is through areaction between the metallic materials in contact and is metallurgicalin nature. A single fabrication process may be used to accomplish boththe bonding and the confining processes. Such confined MIL compositesmay be used to expand the applications and the versatility of MILcomposite materials in structural, armor, and structural-armorapplications. The confined MIL composite materials have excellenttoughness and are effective in stopping and dissipating the energy ofincoming projectiles.

Referring to FIG. 5B, the MIL composite material 501, with either planarcomposite layers or corrugated composite layers, is shown in a crosssectional view to be in contact with four metallic walls 510, 520, 530,and 540 of the box. The box as shown is rectangular in shape and may bein other alternative geometries such as a closed cylindrical shape.

The following describes one exemplary fabrication process for makingsuch a confined MIL composite material where the MIL composite materialis assumed to be a Ti/Al3Ti MIL composite, and the metallic walls of thebox are assumed to be made of titanium as an example but may be any ofthe various first metals.

In preparation, the sheets and plates of titanium and aluminum arecleaned by an appropriate method, e.g., mechanical brushing orhydrofluoric acid bath. Sheets of Ti and Al are then interleaved to forma stack. The stacking order is such that aluminum makes the top most andthe bottom most layers of the stack. Alternatively, titanium sheets andplates may be replaced by any other metal such as nickel, iron, nitinoletc.

The Ti plates may be welded together to form a box with an opening onone side. The inner dimensions of the box are designed to be close tothe dimensions of the stack of the Ti and Al sheets. The stack ofcleaned Ti and Al sheets are placed inside the box. The number of sheetsis selected to make the height of the stack slightly greater than theheight of the box. The box is then closed by pressing down a lid Tiplate on top of the stack. The lid plate is then welded the rest of thebox.

If desired, a small tube may also be welded to the box such that the boxcan be evacuated or back filled with an inert gas. Alternatively, theprocess of placing the stack in the box and welding of the box may beconducted within an evacuated chamber.

Upon sealing the stack in the box, the entire assembly is heated in abox furnace through a specific time-temperature routine to allow thesheets to react to form the MIL composite as well as the metallurgicalbond to the box in the process. The result is a confined MIL compositethat is metallurgically bonded to the box.

Such a confined MIL composite material block may be used as a buildingblock for various structures. Multiples of confined MIL compositematerial blocks may be jointed together to construct large structures.Since the external surfaces of the block is a metal (e.g., Ti), asuitable technique for joining two metal parts may be used to joindifferent confined MIL composite material blocks. For example, weldingmay be used to joint two blocks together. Large armor panels in variousshapes may be constructed from joined blocks.

In some applications, a surface of a metallic substrate, plate, or partmay be coated with a hard layer to provide improved surface hardness,strength, and resistance to wear and corrosion. A MIL compositestructure and the associated fabrication process may be used to formsuch a hard layer. The MIL composite layer is designed to have ahardness greater than the hardness of the substrate to which it isbonded, and the specific hardness can be tailored by suitable choice ofmaterials. The hardened surface layer may include layers of, forexample, titanium-trialuminide or titanium metal andtitanium-trialuminide or titanium metal and titanium trialuminideinterspersed with another hard ceramic, such as boron carbide, siliconcarbide, tungsten carbide, aluminum oxide, silicon dioxide, or anynumber of other hard ceramic materials or interdispersed with metallicparticles of elements such as: titanium, tungsten, nickel, iron, copper,or any number of other metals and their alloys.

In one implementation, a hardened surface layer may be formed on ametallic substrate such as, but not limited to, titanium and its alloys.The hardened layer may exist on either, any, or all of the exposed orunexposed surface of the substrate. This hardened layer may be a layeredstructure. As an example, FIG. 6 shows that the layered structure mayinclude alternating layers of titanium (or a titanium alloy), and layersof an intermetallic phase that is generally titanium-trialuminide. Thehardness of the intermetallic layer is greater than the hardness of thesubstrate, while the hardness of the titanium (or the titanium alloy)layer can be the same, greater, or less than that of the substrate. Thetitanium (or titanium alloy) in the layered structure may be replaced byother metals such as nickel, various kinds of steels etc., in which casethe intermetallic layer formed will be an aluminide of this other metal(i.e. an aluminide of nickel, aluminide of iron etc). In anothervariation, the titanium trialuminide layer may be interspersed withregions of other hard phases such as ceramics. The coefficient ofthermal expansion of the various layers in the coating can be carefullymatched to that of the substrate layer through the use of variouscombinations of ceramic materials and/or metals in the intermetalliclayer(s) to aid in achieving a good bond with the substrate material.

Alternatively, the hardened layer may not be a layered structure and maybe a single layer of titanium trialuminide that is metallurgicallyformed on the Ti substrate.

The choice of coating material combination, e.g., the use ofTitanium/Titanium-trialuminide as the coating material, may be used on asubstrate of a material other than titanium. For example, the substratemay be a nickel alloy and the coating may be a MIL composite layerhaving interleaved Titanium and Titanium-trialuminide layers. As anotherexample, the substrate may be titanium and the coating may be a MILcomposite layer having interleaved nickel and nickel-trialuminidelayers.

In fabrication of a hard surface layer on a substrate, alternate layersof titanium and aluminum sheets of pre-determined thickness are firstplaced on the titanium substrate. As an example, thickness of thetitanium and aluminum sheets may range from about 0.001″ to about 0.1″.In some implementations, the thickness of all the titanium sheets andthat of all the aluminum sheets may be respectively equal to each other,although the thickness of the sheets can vary within the stackingsequence in order to change the resulting hardened layer thicknesssequence. The length and the width of all the sheets may be sized tocover a selected portion or the entirety of the substrate.

Next in fabrication, the sheets are pressed under pressure and heat inthe ambient air as described with reference to the system shown in FIG.2. The reaction between the metals on the substrate surface creates aceramic-like intermetallic layer or a MIL composite layer. Thisfabrication in the ambient air does not need a processing chamber with avacuum system and can be generally cheaper and easier than other layerdeposition methods such as vapor deposition, sputter coating, casehardening which often require specialized environments and equipment. Inanother aspect, the present processing technique may be used to form awide range of thickness of the hard layer. The thickness of the hardlayer in our process is orders of magnitudes greater than that achievedby the conventional techniques such as vapor deposition, sputtercoating, case hardening etc. The process also may be used to provide theflexibility of varying the hardness distribution of the surface layer byutilizing the interspersed material of varying hardness.

A MIL composite layer as the hard surface on a substrate may be furtherconfigured to include various features to improve the performance of thehard layer.

For example, the thickness of the titanium and the aluminum layers maybe spatially graded so that the thickness of each titanium layer may bedifferent from the thickness of other titanium layers in the stack, andthat the thickness of each aluminum layer may be different from thethickness of other aluminum layers in the stack. This graded layerstructure may be used to fine tune the hardness to or close to a desiredhardness value.

As another example, the MIL composite layer may be filled withperforation patterns or holes embedded with suitable hard materials tofurther improve the hardness of the layer. FIG. 7 illustrates oneexample where perforated aluminum sheets with different kinds ofperforation patterns are used to form the MIL composite material and theperforations are filled by either a hard ceramic or another metal.

When the perforations are filled with ceramics, such ceramics mayinclude boron carbide, tungsten carbide, silicon carbide etc. FIGS. 8Aand 8B show photographs of the top surface view and cross-sectionalview, respectively, of an exemplary wear and corrosion coating made fromperforated aluminum sheets filled with boron-carbide powder. Becauseeach perforated aluminum sheet is sandwiched between two Ti sheetsbefore fabrication, cavities are formed in the intermetallic layer afterthe fabrication and are filled with the ceramic materials infiltratedwith Al or the intermetallic phase. FIG. 8C shows an enlarged view ofthe cross-sectional view of FIG. 8B to illustrate examples of theceramic-filled cavities.

A metal substrate coated with hard layer may also be strengthened byhaving vertical hard metal wires embedded in the hard layer and thesubstrate. FIGS. 9A, 9B, and 9C illustrate one example of a wiretoughened substrate coated with a MIL composite layer. Before theprocessing under head and pressure to form the hard layer, holes may beformed, e.g., drilled, into the aluminum and titanium sheets for formingthe MIL composite layer and similar matching holes are also formed inthe substrate. Titanium wires of similar diameter as that of the holesare inserted through the sheets into the substrate. Upon completion ofprocessing, the Ti wires are metallurgically bonded to the MIL compositelayer and the substrate. FIG. 9A shows a top-surface photograph of asample fabricated with Ti wires connecting the substrate plate and thehardened surface layer. FIG. 9B shows a cross-section photograph of asample fabricated with Ti wires connecting the substrate plate and thehardened surface layer. FIG. 9C shows a micrograph of the cross-sectionof one of the Ti wires.

The wired toughened structure may be fabricated in the following processin one implementation. The substrate, with alternating layers oftitanium and aluminum sheets placed on its surface, is placed in betweentwo heater platens, e.g., the system in FIG. 2. These platens arecompressed between the constraints of a load frame with mechanicalfixtures designed to distribute the load uniformly across the substratearea. The entire assembly is then reacted in a load-temperature cycle.Upon the completion of the reaction, the assembly is cooled gradually tominimize separation of the layers due to thermal expansion mismatchbetween the various constituents of the assembly. The final structureincludes the substrate, un-reacted titanium layers, and titaniumtrialuminide layers. In the case of a decorated composite sample, thetitanium trialuminide layer also includes embedded ceramic within theembedding pattern.

In another aspect, MIL composite layers may be structured to includevibration-damping cavities filled with loose powder materials to absorbvibration energy. This damping mechanism is passive and is built into aMIL composite material structure. Hence, the so fabricated MIL compositematerial can inherently damp vibrations. In one implementation, suchdamping cavities may be spatially distributed at positions of highamplitude displacement of vibrations in a MIL composite material. Forexample, titanium (metal)-Titanium Trialuminide (intermetallic)composites may be designed with such cavities to achieve enhancedvibration damping properties in specific vibration modes whilesimultaneously possessing high strength, high toughness, low density andgood corrosion resistance.

FIGS. 10A, 10B, and 10C illustrate one implementation of the fabricationprocess for making a MIL composite material with damping cavities.First, the vibration modes for a give MIL composite material aredetermined. Damping cavities are designed to be located at positions ofhigh amplitude displacement of these vibration modes. Holes of a desiredsize are formed, e.g., by drilling, in titanium and aluminum sheets atsuch pre-determined locations. The drilled layers are then stacked ontop of each other to create a pattern of cylindrical cavities. FIG. 10Aillustrates one example of a MIL composite material with marked holelocations. Two differently sized holes are shown. The drilled layers maybe a 3-layer structure by stacking Al/Ti/Al on top of each other. Next,a titanium ring of the same outer diameter as the cavity is pushed intothe cavity. The height of the ring is made to be the same as the heightof the cavity. Alternatively, Ti cups may be used to replace the rings.Furthermore, rings or cups may be eliminated and “bare” hole are used toreceive the powder material.

FIG. 10B shows that separate stacks of Ti/Al/Ti sheets without any holesare provided and are reacted under load and at a high temperature withina load frame to form MIL composite plates of Ti/Al₃Ti/Ti. Notably, theexternal layers of the plates are Ti layers in order to metallurgicallybond with the external Al layers of the drilled stack of Ai/Ti/Al layersto form an intermetallic layer. Next, the drilled stack of Al/Ti/Allayers is placed on top of one MIL composite plate made in FIG. 10B. Asuitable granular material is filled into the cavities in desired volumefraction. Examples of the granular material include but are not limitedto tungsten carbide, solid glass spheres, titanium diboride, aluminumoxide, and silicon carbide. The holes are then covered up with anotherMIL composite plate. As illustrated in FIG. 10C, this entire assembly isprocessed under pressure and heat to cause reaction between thecontacted layers of the drilled stack and the two MIL composite plates.Optionally, Ti sticks or strips may be placed with each hole to providesupport of the layers above and below the hole.

The above and other MIL composite structures may incorporate sensorsembedded or buried within each structure so that a condition or behaviorof the structure may be measured and monitored. Such sensors providematerial “intelligence” of a given structure. In addition, a controlmechanism may be implemented in such an intelligent material structureto control a condition or behavior of the structure and the sensors andthe control mechanism may be connected to form a control feedback loopso that the entire structure may be intelligently self-controlled. Suchmaterials are smart and multifunctional materials. Various usefulfunctions may be implemented within the materials such as damagedetection, health monitoring, temperature sensing, actuation etc.

As an example, a piezoelectric sensor may operate as both a source and areceiver of ultrasonic pulses, a network of such embedded sensors may beused to determine the location and extent of internal structural damage.Such a network could also be used to locate external impacts on thestructures, such as those caused by projectiles. Further, piezoelectriccrystals may be used to dissipate vibration energy as electrical energyby connecting them in an electrical circuit.

FIG. 11A illustrates one example of an intelligent MIL compositematerial where sensors 1110 are embedded at selected locations. Forexample, vibration sensors 1110 are distributed in the top and thebottom intermetallic layers of the material to measure any impact andthe distribution of the impact. FIG. 11B further shows that eachembedded sensor 1110 may include conductive wires to output anelectrical signal that represents the magnitude of the impact at thelocation of the sensor. Various vibration sensors may be used, includingpiezo-electric sensors that generate electrical signals in response tocompression applied to the sensors.

Embedding sensors within metallic materials such as a MIL compositematerial, however, may experience a high temperature during fabricationprocess that can exceed 500° C. and may reach the range of about 1000°C. A Ti/Al based MIL composite is fabricated by reacting alternatelayers of Ti and Al between 660° C. and 750° C. with the reactionoccurring over typically 6-8 hours. The Al reacts with Ti to form Al3Tiresulting in a final structure comprising alternate layers of Ti andAl3Ti. Piezoelectric sensors embedded within the MIL composites, hence,should sustain the integrity and function at such high temperatures.Lithium-niobate piezoelectric sensors may be designed to operate at suchtemperatures. Other sensors may also be used.

FIGS. 12A and 12B show one example of a MIL composite material with anembedded piezoelectric sensor. In this particular example, the basicmaterials used in the fabrication of the MIL composite may be 0.020″thick Ti-6-4 alloy sheets and 0.024″ thick 1100 Al alloy sheets. Thefabrication of the composite with embedded piezoelectric sensors may becarried in the following five processing steps. First, several Ti-6-4alloy sheets and 1100 Al alloy sheets are stacked alternately andreacted under pressure and heat to form two MIL composite plates, eachapproximately 0.325″ thick. In the second step, four 0.75″ diameterholes corresponding to 4 sensors are machined out in another stack(total height ˜0.150″) of alternately placed Ti-6-4 sheets and 1100 Alalloy sheets. A close-fitting titanium ring, with a hole machined in itswall, is placed in each of the four holes in the sheets. Rectangularslots are machined in the sheets, adjacent to each of the holes and asteel tube are placed in the slot. One end of the steel tube is insertedinto the hole in the titanium ring, while the other end of the tubeextended outside the stack. Thus, at the end of the second step, anun-reacted stack of Ti and Al sheets with titanium rings is provided andsteel tubes are placed in their appropriate locations.

Next in the third step, the assembly of the second step is compressed ina load frame at about 600° C. for several hours and is then cooled down.In the fourth step, the piezoelectric sensors are carefully placed intheir respective holes and their lead wires are passed through analumina tube placed within the metal tube. The sensors are then sealedin the cavity using a high-temperature cement paste (Ceramabond 571,Aremco Products, USA) which is then cured for several hours at roomtemperature followed by curing at about 100° C. Finally, one pre-reactedplate (from step 1) is placed on each side of the stack containing thesensors (step 4), and then processed in the same manner as in step 1.

In one implementation, the piezoelectric sensors may be a 36° Y-cutLiNbO3 crystals from Boston Piezo-optics (Massachusetts, USA). The Curietemperature for the crystals is about 1200° C. and is higher than themaximum processing temperature of about 700° C. in fabricating the MILcomposite material. The crystals may be sized to be 0.5″ in diameter and0.078″ tall with a co-axial electrode pattern.

Only a few implementations are described. However, otherimplementations, variations and enhancements may be made.

1. A method for manufacturing an article, comprising: providing asubstrate made of a first metal and having a surface; placing a metalsheet made of a second metal on the substrate in contact with at least aportion of the surface; applying pressure and heat to the substrate andthe metal sheet to compress the metal sheet against the surface to causereaction between the metal sheet and the surface and to form anintermetallic compound, wherein the metal sheet comprises perforationsto form cavities in the intermetallic compound; and filling the cavitiesin the intermetallic compound with a material to improve hardness of theintermetallic compound.
 2. The method as in claim 1, further comprising:placing a stack of alternating metal sheets of the second metal and thefirst metal on the metal sheet wherein the metal sheet is in contactwith a sheet of the stack made of the first metal; and applying pressureand heat to the stack and the substrate to compress the stack, the metalsheet against the surface to cause reaction between the first and thesecond metals to form an intermetallic compound.
 3. The method as inclaim 1, wherein the first metal comprises titanium.
 4. The method as inclaim 2, wherein the second metal comprises aluminum.
 5. The method asin claim 1, wherein the first metal comprises a titanium alloy.
 6. Themethod as in claim 2, wherein the first metal comprises titanium and thesecond metal comprises aluminum.
 7. The method as in claim 1, whereinthe first metal comprises nickel.
 8. The method as in claim 1, whereinthe first metal comprises a nickel alloy.
 9. The method as in claim 1,wherein the first metal comprises vanadium.
 10. The method as in claim1, wherein the first metal comprises a vanadium alloy.
 11. The method asin claim 1, wherein the first metal comprises iron.
 12. The method as inclaim 1, wherein the first metal comprises an iron alloy.
 13. The methodas in claim 1, wherein the first metal comprises tantalum.
 14. Themethod as in claim 1, wherein the first metal comprises a tantalumalloy.
 15. The method as in claim 1, wherein the first metal comprisesan aluminide-forming metal or alloy.
 16. The method as in claim 2,wherein the second metal comprises an aluminum alloy.
 17. The method asin claim 2, wherein the second metal comprises an aluminum metal matrixcomposite.
 18. The method as in claim 2, wherein the second metalcomprises an aluminum-infiltrate ceramic composite.
 19. The method as inclaim 1, wherein the first metal comprises a metal or alloy that formsintermetallic compounds with magnesium, including aluminum and itsalloys.
 20. The method as in claim 2, wherein the second metal comprisesmagnesium.
 21. The method as in claim 2, wherein the second metalcomprises a magnesium alloy.
 22. The method as in claim 2, wherein thesecond metal comprises a magnesium metal matrix composite.
 23. Themethod as in claim 2, wherein the second metal comprises amagnesium-infiltrate ceramic composite.
 24. The method as in claim 1,wherein filling the cavities in the intermetallic compound with amaterial to improve hardness of the intermetallic compound comprises:filling the cavities with ceramics material.
 25. The method as in claim24, wherein the ceramics material comprises boron carbide.
 26. Themethod as in claim 24, wherein the ceramics material comprises tungsten.27. The method as in claim 24, wherein the ceramics material comprisescarbide.
 28. The method as in claim 24, wherein the ceramics materialcomprises silicon carbide.
 29. The method as in claim 1, wherein fillingthe cavities in the intermetallic compound with a material to improvehardness of the intermetallic compound comprises: filling the cavitieswith metal.
 30. The method as in claim 1, wherein filling the cavitiesin the intermetallic compound with a material to improve hardness of theintermetallic compound comprises: filling the cavities with ceramicsmaterial infiltrated with aluminum.
 31. The method as in claim 1,wherein filling the cavities in the intermetallic compound with amaterial to improve hardness of the intermetallic compound comprises:filling the cavities with ceramics material infiltrated withintermetallic compound.